Copenhagen Metro has a driver-less rail system that runs 24 hours a day. Sao Paulo Metro has a comparatively small 74 kilometers of track, yet carries 3.3 million passengers each day. Montreal has one of the lowest carbon footprints for metro networks in the world.

But what other systems rank among "the best?"

1. Hong Kong

Hong Kong's MTR (mass transit railway) is renowned for being clean and fast.

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Praise for Hong Kong's MTR gushes from every traveler who's ever set a toe inside the immaculately clean, well-signposted, cheap, regular, convenient system that connects most corners of the city, from the crowded bowels of Wan Chai to the rural(ish) villages of Tai Po.

There's free Wi-Fi in every station in Hong Kong, facilities such as tactile flooring and Braille plates for travelers with disabilities and public washrooms, shops, banks and takeaway food outlets inside many stations or close to their exits.

There's no timetable for commuters -- trains just turn up every few minutes, sooner during peak periods -- and it's dead simple for visitors to buy a ticket via the automated machines.

It has possibly the world's most convenient Airport Express service, with departures every 10 minutes or so.

And then there's the Octopus card -- possibly the world's greatest transport payment system, which can also be used in convenience stores, restaurants and other places. Cities such as Melbourne should scream with jealousy. The website offers handy one-day itineraries for Hong Kong tourists keen to shop, eat or discover the local culture.

Occasionally the walk from one concourse to the next can be lengthy and some stations get ridiculously crowded during peak hours (looking at you TST, Causeway Bay). But that just gives more time to watch and wonder as this 211-kilometer, 150-station system copes easily with its 3.4 million passengers every day.

2. Seoul

Seoul's metro system has TV, heated seats and cell phone service.

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Operated by three different companies (two of them state-funded), the Seoul metro system carries almost 7 million passengers per day on nine lines.

In addition to being one of the only metro systems in the world with cell phone service and Wi-Fi, many of the subway trains in Seoul are outfitted with TVs and are climate controlled. We love the toasty, heated seats in the winter.

"Many subway planners come to (South) Korea and are really blown away by the technology that we have in place," says Jung-whan Kim from Seoul Metro's media team. "It's a big showpiece for (South) Korea's emphasis on IT."

The only downside is the early closing time -- around midnight on weekdays, a little earlier on weekends -- considering how obsessed the city is with nightlife.

3. Singapore

The Singapore Mass Rapid transit (SMRT) is probably the fastest way to explore the city.

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After an amalgamation of several transit-service-providers in 2000, SMRT has grown to 600 million passengers per year. Some use it to seek refuge from the heat outside, lapping up the air-conditioned comfort.

The system gets demerits for lack of EZ-Link ticket card machines at some stations, meaning frequent lengthy lines for travelers needing to top up or buy a ticket.

4. London

The London Underground was the world's first underground railway, opened in 1863.

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The London Tube was the world's first underground metro, opening in 1863 and they've not done terribly much since. Still, air-conditioned carriages been introduced, alongside intermittant Wi-Fi signal. But for history, for great underground busking and for something relatively cheap in an expensive city, it's hard to beat.

Despite all the grumbling, the Underground ferries more than 1 billion journeys per year. Not bad for something that old.

5. Paris

Roughly 4.5 million people travel on the Paris metro daily.

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The City of Light's metro is unusually dense, with 245 stations on 14 lines, in just 87 square kilometers of the city. Parisians, apparently, don't like to walk.

With more than 1.5 billion passengers a year, Paris Metro is in the top-five for busiest city-rail services in the world. The Paris Metro does lose some points for not having automatically opening doors. This hints at the average age of the carriages and suggests a need to spend a little on upgrades.

6. Madrid

A giant mural of Spanish flamenco artist Paco de Lucia can be found at Madrid's Paco de Lucia station on the Line 9 of the metro.

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At 294 kilometers, Madrid has the sixth-longest metro system in the world. But on top of that is another 386 kilometers of suburban rail services. All up, Madrid's railway serves 1.5 billion passengers each year with 21 lines and 396 stations. Impressive, particularly given that Madrid's population is only 6.5 million.

7. New York City

On New York's subway, you can actually get arrested for putting your feet on a seat.

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It would be grossly unjust to leave out the city whose subway system, at least, never sleeps. New York City's MTA subway lines are doubled up so all local and express trains can run simultaneously along the same routes, 24 hours a day. And even when carrying out major work on a line, only a single track is decommissioned, leaving a reduced but still-open service. That's planning.

Extra points must go to New York City for the MTA's Music Under New York program, supporting local musicians. Speaking of musicians, a trip on the MTA isn't complete without encountering a busker (or beggar) doing the rounds of carriages.

8. Tokyo

Here, a Shinkansen bullet train waits at Tokyo Train Station. The Shinkansen is a network of high-speed railway lines in Japan.

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Tokyo's rail system is legendary. Super-fast, super-punctual, super-everything. Some 102 train lines, an estimated 14 billion passengers per year. By most measures, Tokyo should take first place on anyone's list of best metros.

Successfully navigating Tokyo by train (and working out which station exit to use) is a proud moment for any traveler. For some a positive, others a negative, you risk social-pariah status if you ever talk on your phone while moving. That's consideration for others at its best. Or worst.

9. Guangzhou, China

Guangzhou didn't have a metro until 1997, now it also has a high-speed train to Beijing, pictured here.

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After failing five times in 30 years to create a metro system, Guangzhou's first metro line was finally opened in 1997 and a second line was opened in 2002. Infrastructure investment exploded in 2004 when the city was awarded the 2010 Asian Games. In the ensuing six years, the council spent $11 billion (RMB 70 billion) on the metro system.

For going from absolute zero in 1992 to eight lines, 144 stations, 236 kilometers of track and 1.2 billion passengers in 2008, and for the 48-minute express-trip to Hong Kong (which opened in 2015), Guangzhou must get a mention.

Editor's note: This article was previously published in 2013. It was reformatted and republished in 2017.